In recent years, an artificial photosynthesis technique of electrochemically converting sunlight into a chemical substance in imitation of photosynthesis of plants has been under development from viewpoints of energy problems and environmental problems. The case of converting the sunlight into the chemical substance and storing it in a cylinder or tank has advantages that the storage cost of energy can be reduced and the storage loss is small as compared to the case of converting the sunlight into electricity and storing it in storage batteries.
As an electrochemical reaction device that electrochemically converts the sunlight into a chemical substance, there has been known, for example, a two-electrode type device that includes an electrode having a reduction catalyst for reducing carbon dioxide (CO2) and an electrode having an oxidation catalyst for oxidizing water (H2O), and in which these electrodes are immersed in water where CO2 is dissolved. These electrodes are connected to each other via an electric wire or the like. In the electrode having the oxidation catalyst, H2O is oxidized by light energy, and thereby oxygen (½O2) is obtained and a potential is obtained. In the electrode having the reduction catalyst, CO2 is reduced and formic acid (HCOOH) or the like is produced by obtaining a potential from the electrode causing the oxidation reaction. In the two-electrode type device, since a reduction potential of CO2 is obtained by two-stage excitation, a conversion efficiency from the sunlight to chemical energy is about 0.04%. Besides, for example, there has been known a device in which the photoelectric conversion is performed by using GaN, water is oxidized at a surface thereof, and CO2 is reduced by using a copper plate that is connected thereto as the electrochemical reaction device. A conversion efficiency of the above-described device is 0.2%.
An electrochemical reaction device using a stack (silicon solar cell or the like) where a photoelectric conversion layer is sandwiched between a pair of electrodes has been also under consideration. In the electrode on a light irradiation side, water (2H2O) is oxidized by light energy, and thereby oxygen (O2) and hydrogen ions (4H+) are obtained. In the opposite electrode, by using the hydrogen ions (4H+) produced in the electrode on the light irradiation side and potential (e−) generated in the photoelectric conversion layer, hydrogen (2H2) or the like is obtained as the chemical substance. Besides, there has been also known an electrochemical reaction device where silicon solar cells are stacked. The electrochemical reaction device preferably has a high conversion efficiency.
Causing reaction using, as an electrolytic solution, seawater, river water, or the like, which is present in large amounts, has been under consideration from the environmental aspect or the like. However, by using, for example, the river water or the like as an electrolytic solution, activity of a catalyst decreases due to impurities or the like, resulting in a decrease in conversion efficiency. Further, using an electrolytic solution having an extremely high pH or low pH causes a problem that members to be used deteriorate, or the like. As above, in the conventional electrochemical reaction device, kinds of usable electrolytic solutions are limited. The cost is high due to environmental restriction and limited members. Accordingly, there has been a problem of low versatility.